It is common practice in video production to use a fade region to start or end a scene. When such a region is used at the beginning of a scene it is called fade-in, but when it is used at the end of a scene it is called fade-out. In a fade-in region, the scene gradually appears while a solid color such as black or white gradually fades out, and by the end of this region, the scene completely replaces the solid color. Similarly, in a fade-out region the scene gradually fades out, while a solid color such as black or white fades in, and by the end of this region the solid color completely replaces the scene. Although a fade region can be up to several seconds long, fades of 1/2 to 1 second are more common in video production. When the same process occurs between two different scenes it is called a dissolve. Although fades can be considered a special case of dissolves, only fades are addressed in this application.
Attempting to compress a fade region via image compression using a typical motion compensation algorithm results in a high entropy prediction error image. Encoding such an image requires more bits than usual, which may not be affordable for all of the fade region without increasing the average bit-rate. Furthermore, an image that is encoded with an insufficient number of bits appears blocky when it is decoded. Similar problems occur whenever a scene contains a rapid but non-uniform change in the overall luminosity from one frame to another due to an explosion or a flash bulb. In this application, such a luminosity change will also be referred to as a fade. The blockiness due to the luminosity difference between the images of a scene can be avoided by first detecting those frames with luminosity difference, and then encoding them in a special way.
It is therefore an object of the present invention to provide a system for detecting fades and rapid changes in overall luminosity within digital video sequences.
Further objects and advantages of the invention will become apparent from the description of the invention which follows.